1. Field of the Invention
The present invention relates to an imaging apparatus, an imaging system, its controlling method, and its program and, more particularly, to an offset correcting technique regarding a photographed image.
2. Description of the Related Art
Hitherto, most of photographs have been film photographs (silver salt photographs) obtained by using an optical camera and a silver salt film. In association with the development of a semiconductor technique, an imaging apparatus such as a video camcorder which can photograph a motion image by using a solid-state imaging device using a silicon (Si) single crystalline sensor represented by a CCD type sensor or an MOS type sensor has been being developed. However, an image obtained by such an imaging apparatus using the solid-state imaging device is inferior to the film photograph in terms of the number of pixels and an S/N ratio. The film photograph is generally used in order to photograph a still image.
On the other hand, in recent years, demands for an image processing by a computer, a preservation by an electronic file, a transmission of the image by E-mail, and the like are increasing. An electronic imaging apparatus for outputting a photograph image, as a digital signal, which is not inferior to the film photograph image is demanded. This is true of not only general photographs but also inspection and medical fields.
For example, there is an X-ray radiograph as a photograph using a photograph technique in the medical field. An X-ray generated from an X-ray source is irradiated to an affected part of a human body as an object and the X-ray radiograph is used to discriminate the presence or absence of a fracture of a bone or a tumor or the like based on information of a transmission of the X-ray. The X-ray radiograph is widely used for a medical diagnosis for a long time. Ordinarily, the X-ray transmitted through the affected part enters a phosphor once, is converted into visible light, and thereafter, is exposed to a silver salt film.
However, although the silver salt film has such advantages that a sensitivity and a resolution are high, there is such disadvantages that it is troublesome for development, it takes a time for preservation and management, the film cannot be sent soon to a remote plate, and the like. Therefore, an electron X-ray imaging apparatus for outputting the photograph image, as a digital signal, which is not inferior to the film photograph image as mentioned above is demanded. This is true of not only the medical field but also a non-destructive inspection field of a specimen such as a structure or the like.
To meet such a demand, an imaging apparatus (Flat Panel Detector; hereinbelow, abbreviated to “FPD) using a large-scale sensor obtained by two-dimensionally arranging imaging devices having photoelectric conversion elements made of amorphous silicon hydride has been being developed. The FPD is realized by using such a principle that when an electric field in the reverse direction is applied to the photoelectric conversion element, a photoelectric current according to a quantity of incident light flows in a semiconductor layer.
According to the FPD, for example, a metal layer, an amorphous silicon layer, or the like is deposited onto an insulating substrate whose one side is equal to about 30 to 50 cm by using a sputtering apparatus, a chemical vapor evaporation deposition apparatus (CVD apparatus), or the like and the photoelectric conversion elements (photodiodes) and thin film transistors (hereinbelow, abbreviated to “TFTs”) are formed. Thus, for example, about (2000×2000) photoelectric conversion elements are formed, the electric field of a reverse bias is applied to them, and, at the same time, charges flowing in the reverse direction of each of the photoelectric conversion elements can be individually detected by the formed TFTs.
However, according to the FPD, a current called a dark current flows even in a state where no light is irradiated, thereby causing an artifact in the image. Further, the dark current exerts an influence as a shot noise on the image and becomes one of factors which cause detecting ability, that is, a sensitivity (S/N ratio) of the whole apparatus to be deteriorated. There is a case where such a deterioration exercises an adverse influence on the medical diagnosis and the judgment about the inspection. For example, naturally, a problem occurs if one of a focus and a defective part is overlooked due to the shot noise as a cause. It is, therefore, important to reduce the dark current as much as possible.